1. Field of the Invention
This invention generally relates to a digital signal recording apparatus, and particularly relates to an apparatus for recording a digital signal and an amble pattern signal on a magnetic recording medium. This invention also relates to a method of recording a digital signal on a recording medium.
2. Description of the Related Art
In some digital VTR's (video tape recorders), a digital information signal is recorded on a magnetic tape while an array of slant tracks is formed thereon. Each slant tracks is divided into areas assigned to main information pieces (for example, video information pieces, audio information pieces, subcode information pieces, and digital data pieces), and areas assigned to signals representing amble patterns such as a preamble pattern and a post-amble pattern. The main-information areas are separated by the amble-pattern areas.
The signals recorded on the amble-pattern areas are designed to decide the phase of a clock signal derived from reproduced data, to enable the absorption of errors in signal record positions during an editing process, and to compensate for an error in a track reproduction position due to a positional head attachment error.
In a typical digital VTR, a signal reproduced from a magnetic tape by magnetic heads is subjected to an equalization process by a waveform equalizer, and the equalization-resultant signal is converted into a binary digital signal (a bi-value digital signal). Then, a PLL (phase locked loop) circuit derives a clock signal from the binary digital signal. Accordingly, it is desirable that signals recorded on amble-pattern areas have such lengths as to stabilize the data derivation phase provided by the PLL circuit, and that the signals recorded on the amble-pattern areas provide such data inversion intervals as to easily enable the derivation of the clock signal.
In general, since a PLL circuit responds to data state inversions, an amble-pattern signal, which has many data state inversions during a given time interval, is effective in deriving a clock signal. For example, an amble-pattern signal which represents a bit sequence as "101010 . . . " is the most effective in providing a locked state of the PLL circuit, and in optimizing the phase of the derived clock signal.
It is known to use partial responses in magnetic recording and reproducing apparatuses. During the reproduction of a digital signal from a magnetic recording medium, the digital signal is differentiated by a magnetic head so that the reproduced signal outputted from the magnetic head basically has a differentiation-resultant waveform. Since a reproducing device including the magnetic head has a narrow passband width, the actual waveform of the reproduced signal agrees with a rounded or smoothed differentiation-resultant waveform.
According to a partial-response detection system "PR(1)" based on integral detection, the center level of the waveform of the output signal from the magnetic head is used to decide whether a reproduced signal state corresponds to a logic state of "1" or a logic state of "0". When a bit sequence represented by a recorded signal on a magnetic recording medium is denoted by "a.sub.k ", a detected or reproduced signal bit sequence "b.sub.k " provided in the partial-response detection system "PR(1)" is given as "b.sub.k =a.sub.k ". In this case, an amble pattern "101010 . . . " is detected and reproduced as a bit sequence "101010 . . . " which is the most convenient for a PLL circuit to derive a clock signal.
According to a partial-response detection system "PR(1, 0, -1)" better than the partial-response detection system "PR(1)", a recorded signal bit sequence "a.sub.k " and a detected or reproduced signal bit sequence "b.sub.k " are related as follows. EQU b.sub.k =a.sub.k.sym.a.sub.k-2
where ".sym." denotes modulo 2 addition. In this case, an amble pattern "101010 . . . " is detected and reproduced as a bit sequence "100000 . . . " which is inconvenient for a PLL circuit to derive a clock signal.